The present invention relates to the art of step by step switching apparatus as used for example in telephone switching systems. More particularly, the present invention relates to a step-by step switch which has been improved and updated by inclusion therein of an electronic programmed digital microcontroller conversion module.
Almon B. Strowger invented and patented the first two motion or step-by-step switch in 1981. Since that time Strowger switches have been refined and are still used in a widespread basis in telephone automatic switching systems in this country and throughout the world.
Conventional two-motion stepping switches comprise three basic parts. These parts are a pair of contacts banks, the stepping mechanism and the control relays. Typically each bank of contacts contains 100 positions with two metal contacts (for tip and ring wires on one bank and corresponding control wires on the other) at each position. The stepping mechanism uses three electromagnets, a vertical magnet, a rotary magnet, and a release magnet. In conventional telephone parlance the control relays are identified with letters of the alphabet, with selector switch control relays being identified as "A" through "E" and with connector switch control relays being identified as "A" through "J".
A typical 10,000 subscriber telephone exchange implemented with step-by-step switches includes one or more switching trains comprising a "line finder", a "first selector", a "second selector" and a "connector". When a telephone is taken "off hook" by a party about to make a call, the line finder "finds" the line to which the calling party's telephone is connected and electrically connects to that line. Dial tone is then returned to the calling party's instrument. As the calling party dials each number the rotary dial generates a pulse train for each digit dialed. These pulse trains are applied in turn to operate the first selector, the second selector and the connector switches.
For example, in the case of the first selector, the pulses of the first digit dialed are applied to operate the vertical magnet, and each pulse steps the switch shaft up one vertical position, corresponding to a row of ten contacts. The rotary magnet is then operated by pulses generated at the switch in order to find an available path to a subsequent switch. Each such pulse causes the shaft to rotate one step to each contact position of the selected row. Thus, step pulses are effective to lift the wiper of the shaft to the desired row of contacts and then to rotate the wiper to the desired contact position.
At the completion of the call, when the calling party hangs up, ground is removed from the commonly connected control lead, causing the release magnet of each switch in the train to be momentarily energized. This release magnet then releases the shaft from the position reached by operation of the vertical and rotary magnets. Springs apply bias force to the shaft, so that when the release magnet is energized and the shaft is released, it first rotates out of the row of contacts, and then it drops to its original starting location. The switch is then ready to handle the next call assigned to it.
Over the years such step-by-step switches have worked very well and have proven to be quite reliable. Nevertheless, they have certain weak points and drawbacks. For example, the control relays have typically required more attention, maintenance, adjustment and replacement than have the other elements of these switches. Adjustment of the control relays has required the provision of special tools and highly trained technicians.
Another drawback of the control relays as used in these switches has been their inherent limitation as to sensitivity, measured by the resistance of the overall loop length. Typically, connector switches have only been effective to detect when a called party has answered when the overall loop resistance of the communications path did not exceed 1,500 Ohms.
One more drawback of conventional step-by-step switches was that the actual electrical pulses applied through and by the control relays to the vertical, rotary and release magnet varied both in amplitude and in duration, depending upon such factors as the loop resistance to a particular calling party's instrument, the characteristics of the rotary dial of that instrument, and so forth. With deregulation of the telephone interconnect industry, many rotary dial instruments are now being offered for service, and their low cost dial switches require far greater operating margins than heretofore.
Yet another drawback of conventional step-by-step switches was the need for constant cleaning of the contacts to assure that the switches did not become corroded and consequently fail in service. Prior practice called for a maintenance person to wipe all of the contacts of these switches at regular intervals to keep them clean.